The operational WPC Winter Weather Desk (WWD) creates 24-h forecasts of snowfall and
freezing rain accumulations for each of three consecutive 24-h periods (days)
extending 72 hours into the future. These products are shared with the NWS Weather
Forecast Offices (WFO) in a collaborative process resulting in refinement of
the accumulation forecasts. After the 24-h snowfall and freezing rain
accumulation forecasts are finalized, the WWD issues its public products: a
limited suite of
probabilistic
winter weather forecasts. These probabilistic forecasts are computed based on
the deterministic accumulation forecasts combined with ensemble information (see below) and are manually edited by the WWD forecaster.
The probabilistic forecasts found here on the WPC PWPF page are also based on
the deterministic WWD
accumulation forecasts, but are generated totally automatically using an ensemble of
model forecasts along with the WWD forecasts. The automatic nature of this product
generation allows a much more extensive set of displays of probabilities for
snowfall or freezing rain exceeding a number of thresholds and accumulations of snowfall
or freezing rain for various percentile levels.

The automatic processing also allows the generation of probabilistic winter
precipitation forecasts for 48-h intervals based on 48-h accumulations obtained
by adding two 24-h accumulations together. The same method used to compute the 24-h
probabilistic products is applied to the 48-h intervals ending at 48 and 72 hours
after the initial time.

A multi-model ensemble is utilized to create a distribution of values around the
WPC accumulation at each grid point. The typical constituency of this ensemble is as follows:

SLR refers to the snow-to-liquid ratio, which is a multiplicative factor applied
to precipitation accumulated as type snow to compute the snowfall. The 6-h
SLR at each grid point is an average of the value obtained using the
Roebber et al (2007) neural network algorithm (Rnna) applied to the NAM forecast,
the value from the Rnna applied to the GFS forecast, a seasonal climatological value,
and 11. The 24-h mean SLR applied to the GEFS is the average of four 6-h
SLRs covering the 24-h period. For all other members listed above, the 24-h accumulations
are sums of 6-h accumulations, using the 6-h SLR values in the case of snowfall.
The precipitation type determination for the NCEP
models is the dominant type algorithm (Manikin 2005). Precipitation type for non-NCEP models
is determined by applying a
simple decision tree algorithm using surface temperature, and temperatures on the 925-hPa, 850-hPa, and
700-hPa mandatory isobaric levels.

A binormal probability distribution (density) function (PDF), which allows skewness,
is utilized for the PWPF.
The fitting of the binormal distribution is a method of moments approach.
The WPC forecast is the mode of the distribution. The placement of the WPC
forecast in the ensemble order statistics determines the skewness of the distribution.
The variance of the distribution is matched to the variance of the ensemble.
The WPC deterministic forecast is included as a 29th member of the ensemble
for the computation of the variance.
This fit is done at each grid point; so, the probability density function
(PDF) varies from grid point to grid point.

The PWPF forecasts provide information in the following formats:

Probabilities of exceeding a threshold show filled contour levels
of probability that the 24-hour or 48-hour accumulation of winter precipitation will
equal or exceed the given threshold. As an example, consider the
6-inch threshold for snowfall. If a point of interest falls within the 40%
contour on the probability map, then the chance of snowfall
exceeding 6 inches is 40% or greater. As the threshold values
increase, the probabilities of exceeding them decrease.

Percentile accumulations for 24- or 48-hour intervals show filled
contours of snowfall or freezing
rain amounts for which the probability of observing that amount or less is
given by the percentile level. For example, if the 75th percentile map
shows six inches of snow at a location, then the probability of getting up to
six inches of snow is 75% at that point. Conversely, there is only a 25%
probability of snowfall exceeding six inches at the location in this example.
Percentile accumulations increase as the percentile level increases.